Field
The present invention relates to a semiconductor device provided with a group III-V nitride semiconductor film such as nitride gallium (GaN).
Background
A group III-V nitride semiconductor film, AlxGayInzN (x+y+z=1, y≠0) film in particular, has a high saturation electron speed and a high withstand voltage characteristic, and is therefore used as a material for electronic devices. Among these electronic devices, high electron mobility transistors (HEMT) that generate a high concentration two-dimensional electron gas (2DEG: two-dimensional electron gas) on an interface using a hetero-structure are attracting attention in particular. Hereinafter, a HEMT using an AlxGayInzN (x+y÷z=1, y≠0) film will be referred to as “GaN-based HEMT.”
With the GaN-based HEMT, an electron supply layer made of AlxGayInzN (x+y+z=1, x>z) having a larger band gap than an electron traveling layer is provided in an electron traveling layer made of AlxGayInzN (x+y+z=1, y≠0). This makes it possible to generate a high concentration 2DEG through a polarization effect, reduce sheet resistance and obtain high output.
With the GaN-based HEMT, a transition metal such as iron (Fe) or carbon (C) is added to a region lower than the 2DEG region in order to improve a pinch-off characteristic or improve an off-withstand voltage. These dopants form deep levels in the AlxGayInzN (x+y+z=1, y≠0) layer, and have therefore a characteristic of trapping electrons and the doped region becomes a high resistance region. Here, a good pinch-off characteristic means that a leakage current flowing through a drain while applying a gate voltage that causes the transistor to turn off and applying a source-drain voltage (e.g., operating voltage) is sufficiently small. On the other hand, a high off-withstand voltage means that while applying a gate voltage that causes the transistor to turn off and applying a large voltage equal to or higher than the operating voltage between the source and drain, the leakage current flowing through the drain is caused to increase, resulting in a high voltage that may lead to a destruction of elements.
Generally, either a transition metal or carbon is selected as a dopant. Regarding this, JP 5696392 A describes that since the energy level of the transition metal is not stable enough, and so carbon as well as the transition metal needs to be added. On the other hand, JP 5696392 A describes that carbon causes a transient response of a current-voltage characteristic represented by current collapse to deteriorate. Based on this, concentration of carbon is changed at a concentration lower than the concentration of the transition metal as in the case of the concentration change of the transition metal (see FIG. 6 of JP 5696392 A). JP 5696392 A considers that it is thereby possible to stabilize the energy level of the transition metal.
However, the method of JP 5696392 A not only adds C (carbon) to the GaN layer into which the transition metal is intentionally added, but also adds C to the GaN layer above where the transition metal is trapped in a sweeping form. For this reason, C extends to the vicinity of the 2DEG, causing a transit response of current-voltage characteristics such as current collapse to deteriorate. There is also a problem that it is not possible to sufficiently improve both the pinch-off characteristic and the off-withstand voltage. That is, an intrinsic question as to what kind of doping is necessary to improve the pinch-off characteristic and the off-withstand voltage is yet to be defined, failing to become an optimum structure. Furthermore, the amount of added C is generally controlled by growth conditions such as a growth temperature and pressure. Therefore, to gradually lower the concentration of the amount of added C as in the case of the transition metal, it is necessary to realize growth while gradually changing the growth temperature or the pressure. For this reason, not only a huge constraint may be imposed on the growth condition but also the growth is realized while gradually changing the growth speed, making it very difficult to perform film thickness management. Thus, according to the method described in JP 5696392 A, it is not possible to optimize all of the pinch-off characteristic, off-withstand voltage and transient response, and it is also difficult to implement stable production.